This invention relates to the actuation of valves which are connected into tubular elements at the upper end of a drill string, to close off the flow of fluid through the tubular element, either for the purpose of preventing upward flow of fluid from the well under blowout conditions, or to retain a quantity of drilling fluid in a top drive drilling unit or the like when it is detached from the remainder of the string.
Valves of this type at the upper end of a drill string usually include a ball valve element containing a passage through which fluid can flow in one condition of the valve but which is closed when the ball is turned through 90.degree.. There have heretofore been proposals for powered actuation of such a valve by one or more piston and cylinder mechanisms connected to a structure at the upper end of the drill string and coacting with other parts for pivoting the valve between open and closed positions. In one prior arrangement, a rack actuated by a piston and cylinder mechanism engages a pinion attached to the valve to turn it. In another prior expedient, a piston and cylinder mechanism acutates a sleeve about the valve upwardly and downwardly, with the sleeve functioning to swing a crank arm connected to the valve.